Method of preparing metal for painting



Patented Nov. 11, 1930 UNITED STATES PATENT OFFICE JAMES H. GRA'V'ELL,OF ELKINS PARK, PENNSYLVANIA, ASSIGNOR TO AMERICAN CHEMICAL PAINTCOMPANY, OF AMBLER, PENNSYLVANIA, A CORPORATION OF DELAWARE Ho Drawing.

This invention relates to preparing metal for painting or other appliedfinish and, although it has various applications in the art, it isespecially useful for preparing sheet steel I articles, such asautomobile bodies, fenders and stampings, for receiving appliedfinishes,

such as paint, lacquer, apan, varnish, etc.

The objects of the invention are to clean the metal, to prepare it forpainting or other applied finishes, to prevent the development ofdefects in the applied finish, to remove rust and scale from the metal,to retard the v rusting of steel, to provide an acid cleaning processthat will not produce subsequent corrosion, to reduce the cost ofcleaning metal to be painted and generally to improve the art ofpreparing metal for painting.

The term painting as used in the following specification and claims isintended to embrace any applied finish such as is employed for metalsfor the purpose of preservmg or ornamenting the surface thereof and anymethod of applying same.

Generally stated, my process consists in coating the metal with acid,usually phosphoric acid, and then treating the metal with a materialadapted to neutralize the acid.

When phosphoric acid is applied to metal,

such as steel, a certain amount of the metal is dissolved in the acidand when the metal is then treated with a material adapted to neutralizethe acid, the said metal in solution is precipitated as a waterinsoluble material with the result that the metal becomes covered with afilm of this insoluble material, which tends to prevent it from rusting,and, at the same time, affords an excellent surface on which to applyvarious finishes, such as paint, lacquer, apan or varnish.

As phosphoric acid is a relatively expensive acid, I have found thatconsiderable economy can be accomplished by selectively controlling theaction of phosphoric acid so that, although its action in removing rustand scale is unhampered, its action on good metal is materially reduced,thus effecting a large saving in both metal and acid and providing amore uniform film on the metal. Any suitable control agent may be used,but I 50 prefer due to its effectiveness, to employ a METHOD OFPREPARING METAL FOR PAINTING Application filed October 2, 1928. SerialNo. 309,905.

thio-amide, as for instance, thiourea. This I admix with the phosphoricacid in small amfiunts before coating the metal with the am In practiceI have found that much better results are obtained if the neutralizingmaterial is removed from the metal before the paint is applied and thusI deem it advisable to wash the metal with water after performing theinitial steps of my process. This washing with water not only removesexcessive neutralizing material but also removes all deleteriousproducts of its reaction.

When there is danger of the washing operation not being performed withsufiicient care, it is well not to use strong caustics such as causticsoda, tri-sodium phosphate, etc., to neutralize the phosphoric acidcoating as traces of such materials left behind on the metal, mayadversely aflect the painted finishes. In order to overcome thispossible defeet, I prefer to employ a material for neutralizing purposesthat neutralizes the phosphoric acid to a hydrogen phosphate, and yet initself is not caustic, and hence, if any of the neutralizing material orany of the neutral.- ized acid is left behind on the work after thewashing operation, no harm is done. A neutralizing material well suitedfor the purpose is di-sodium phosphate (Na HPO which in itself is not acaustic material, yet will neutralize phosphoric acid to mono-sodiumphosphate (NaH PO and precipitate the metal dissolved in the acid.

The di-sodium phosphate may be used dissolved in water and the samesolution may be used and reused until it is exhausted. In order todetermine when the solution is exhausted it is well to add a smallamount of an acid indicator to the solution so that an indicative colorchange willdevelop as soon as any free acid remains un-neutralized. Ihave found in practice that methyl orange proves to be a verysatisfactory indicator.

It often happens, either through design or accident, that the metal tobe treated has a coating of oil on it, in which case it is difficult tocoat the metal with phosphoric acid. It is accordingly advisable toremove this oil coating before trying to coat the metal with acid. Thismay be accomplished by treating the metal with a caustic soda solutionbefore the acid is applied or by admixing oil removing solvents, such asalcohols, keto-nes, ethers, etc., with the phosphoric acid so as tocause it to coat the metal. However, I consider the most practicalmethod to be to heat the metalto a temperature adapted to remove anycoating of oil before applying the acid.

To carry out my process commercially, as for instance, on an automobilesteel fender I may proceed as follows I first heat the metal to atemperature of'600 F., more or less, to burn off all oil and grease, andafter it has cooled sufficiently, I immerse it in a. bath of phosphoricacidmade by admixing the following materials in substantially theproportions stated:

Water, 10,000 gallons; phosphoric acid (7 5%) 1,000 gallons; thiourea,0.0036 pounds more or less,-' the purpose being to use sufii-- cientacid to properly clean the metal and sufiicient thiourea to selectivelycontrol the action of the acid.

I permit the metal to remain in the bath long enough to clean it tosatisfaction and then remove it. The metal comes from the bath coatedwith phosphoric acid and ready to be treated with the neutralizingmaterial. This is conveniently done by immersing the metal in. a bathhaving a proportional composition substantially of:

Di-sodium phosphate, 1 pound; water, 2 gallons; methyl orange, trace.

I permit the metal to remain in the neutralizing bath long enough toneutralize all freephosphoric acid on the metal, which usually takes aminute or two, more or less. I then remove 1t and lmmerse it in a bathof runnlng water to remove the material earned over from theneutralizing bath, after which I permit the metal to dry. It is thenready for receiving the applied painted finish.

If desired, all baths may be used hot, in order to speed up theoperations, but I prefer to use only the acid bath heated, and to drythe metal which is leaving the water bath at an elevated temperature inan oven.

Metal treated according to my process is free from oil and grease andwill not show a water-break if dipped into water; it has no tendency torust from the after effects of my preparing process and, in fact, willremain for long periods without protection, if not exposed to theweather, without showing signs of rust. When painted finishes, such aspaint, lacquer, japan or varnish is applied to metal prepared by myprocess, it is unusually permanent and does not show development ofdefects. As the process as described may be carried out with the use ofbaths, it is evident that the metal parts to be treated may be hung on acontinuous chain and be made to pass automatically from bath to bath andthus eliminate much handling and the accompanying expense.

Having now described my invention, I claim.:

1. The method of preparing metal for painting, consisting in coating themetal with phosphoric acid and treating the metal with a materialadapted to convert phosphoric acid into a hydrogen phosphate.

2. The method of preparing metal for painting, consisting in coating themetal with phosphoric acid and thereafter treating the metal with anon-caustic neutralizing material. p

3. The method of preparing metal for painting, consisting in coating themetal with phosphoric acid and treating the metal with a materialadapted to convert phosphoric acid into a hydrogen sodium phosphate.

4. The method of preparing metal for painting, consisting in coating themetal with phosphoric acid and thereafter treating the metal withdi-sodium phosphate.

5. The method of preparing metal for painting, consisting in firstremoving any oil from the surface thereof, then coating the .metal withphosphoric acid and thereafter neutralizing the phosphoric acid with anoncaustic material.

6. The method of preparing metal for painting, consisting in heating themetal to a temperature adapted'to remove oil from the surface thereof,coating the metal with phosphoric acid and thereafter treating the metalwith a non-caustic material adapted to neutralize phosphoric acid.

7. The method of preparing metal for painting, consisting in heating themetal to a temperature adapted to remove oil from the surface. thereof,coating the metal with controlled phosphoric acid, treating the metalwith di-sodium phosphate in the presence of an acid indicator andremoving the di-sodium phosphate from the metal together with thedeleterious products of its reaction.

7 Signed at New Yorkin the county of New York and State of New York this1st day of October, A. D. 1928.-

JAMES H. GRAVELL.

